Brilliant thread, I'm assuming that there is no equivalent formula today?
Can you imagine nowadays taking a medium sized family coupe and inserting a current Formula 1 engine into it, in your own garage and then racing it.
MSA Sports Libre classes are about as close as you get generall - basically anything goes. Hillclimbing tends to get the craziest creations these days from what I've seen.
But there is still a specials saloon series
www.specialsaloons.co.uk/